1. Field of the Invention
This invention relates to a tracking servo device employed for an optical apparatus for reproducing the information recorded on an optical disc, or recording the information on the optical disc, such as a recording and/or reproducing apparatus for a magneto-optical disc or a recording and/or reproducing apparatus for a phase-transition optical disc.
2. Description of the Related Art
In a pre-groove formed on, for example, a magneto-optical disc used in a magneto-optical disc apparatus, wobbling signals, as the address information, are sometimes recorded. An optical pickup used for reproducing the information recorded on this sort of the magneto-optical disc or recording the information thereon performs tracking servo in such a manner that a light beam from a light source is radiated on the surface of the magneto-optical disc and a light beam from the magneto-optical disc is received by a light receiving unit segmented into plural sensors. The tracking error signal of the so-called push-pull system is detected based on the difference of the light volumes received by the segmented sensors. Tracking servo is performed by the optical pickup based on this tracking error signal.
Meanwhile, if a wobbled groove having a pre-set amplitude and a pre-set frequency is formed on a data track of the magneto-optical disc, such wobbled frequency components are contained in an output signal of the segmented sensors of the light receiving unit. The amplitude of these wobbled frequency components are varied depending on the position of the objective lens of the optical pickup. This feature may be exploited for detecting the position of the objective lens for canceling an offset produced in a tracking error signal, as proposed in JP Patent Application No.6-285389. The system of canceling the offset value produced in the tracking error signal is termed a wobble push-pull (WPP) method.
The wobble push-pull method, exploiting the wobbled frequency components, cannot be used if the tracking servo is off. If the tracking servo is off, a track-on circuit is used for canceling the offset value of the tracking error signal in place of the WPP circuit, as proposed in WO 95/31806 (International Publication Number).
A changeover circuit is used for switching between these two canceling circuits, that is the wobble push-pull circuit and the track-on circuit. Specifically, the changeover circuit connects to the WPP circuit or to the track-on circuit when the tracking servo is on or off, respectively, for removing the offset value for producing a correct tracking error signal.
The reasons the changeover circuit has to switch between the wobble push-pull circuit and the track-on circuit depending on the tracking servo on/off state are as follows: Since the WPP circuit exploits the wobbled frequency components contained in the detection signal by the optical pickup, it cannot operate correctly unless tracking servo is on and disc rotation is locked to constant linear velocity (CLV). That is, the WPP circuit cannot be used if tracking servo is off, as described above.
Since the track-on circuit holds the offset value of the tracking error at the instant of turning the tracking servo on, the offset value held by the track-on circuit ceases to be correct if the objective lens is moved to follow up with tracks. By these reasons, it becomes necessary for the above-described changeover circuit to switch between the WPP circuit and the track-on circuit depending on the tracking servo on/off state.
FIG. 1 shows an illustrative structure of a WPP circuit 1, a track-on circuit 2 and a changeover circuit 3 as proposed by the present Assignee.
In the switching system of the cancellation circuit, shown in FIG. 1, the changeover circuit 3 switches between the two cancellation circuits, that is the WPP circuit 1 and the track-on circuit 2.
FIG. 2 shows the states of signals of various portions of FIG. 1 since the time when tracking servo so far turned on is turned off at time ts to cause track jump to move an objective lens until the time when tracking servo is again turned on (on and after time t0).
A tracking servo control signal S1 is a control signal indicating tracking servo on/off and turns tracking servo off and on with its high and low states, respectively. A tracking status signal S2 is a signal indicating the off-track state. The tracking status signal S2 is used in a braking circuit for improving recovery characteristics directly after turning on tracking servo at time t0 and also termed a braking pulse. The signal S1 is low or high when the light from the light source is on-track or off-track, respectively.
A cancellation circuit changeover signal S5 operates so that, during the time when tracking servo is off (since time point ts until time point t0) and during the time when tracking servo is on, the braking pulse (off-track status signal S2) subsides and the WPP circuit 1 again operates correctly (since time point tz until time point tu), an output of the track-on circuit 2 becomes a tracking error signal TE, and so that, during the time other than the time periods specified above, an output of the WPP circuit 1 will become the tracking error signal TE.
A track-on hold signal S6 is a signal for hold/release switching of the offset value of the tracking error signal TE as detected by the track-on circuit 2, and sets the hold state since a time instant when the tracking servo is turned on until the tracking servo is next turned off. This track-on hold signal S6 is coincident with the tracking servo control signal S1.
A WPP cancellation signal S9 of FIG. 2 is a cancellation signal in the WPP circuit 1 of FIG. 1 (equivalent to the second term of the right side of the following equation (1) for the tracking error signal: ##EQU1##
As long as tracking servo is on, the WPP cancellation signal S9 indicates a correct cancellation value A. However, during track jump, (during tracking servo off time, that is the time since time point ts until time point t0), there is no correct wobbling signal present in the WPP circuit, because tracking servo is off. Further, depending on the jump speed, the traverse signal of the tracking error signal is sometimes degraded to the wobbled frequency, in which case the cancellation value becomes an incorrect value.
Tracking servo is then turned on at time point t0, from its off state, so that a correct wobbling signal enters the WPP circuit 1. However, because of the long time constant of a low-pass filter LPF of wobbling amplitude detector 4 (FIG. 1), a long delay time t1 has to elapse until the WPP cancellation signal S9 reaches a correct cancellation value B. This delay time t1 is on the order of, for example, five milliseconds. That is, the delay time t1 has to elapse since the tracking servo, so far turned off, is turned on and rotation of the magneto-optical disc becomes locked to CLV until the cancellation value of the WPP cancellation signal S9 of the WPP circuit 1 becomes a correct value.
It should be noted that an output S10 of the WPP circuit is simultaneously an output of the WPP circuit 1 and an input to the track-on circuit 2. However, the track-on circuit remains unaffected as long as the track-on circuit 2 is selected by the cancellation circuit changeover signal S5, since then the changeover switch 6 overrides WPP cancellation signal S9.
A track-on circuit output S11 is controlled so that, as long as tracking servo is on, an offset value of a tracking error signal detected by the track-on circuit 2 when the tracking servo is changed over from its off state to its on state is held by the tracking hold signal S6. Specifically, the potential of a capacitor for holding the track-on circuit 2 should not be changed. However, in effect, minor current inevitably flows through this capacitor to change the capacitor potential. Moreover, the time during which the tracking servo is on is significantly longer than the time during which the tracking servo is off, so that the amount of deviation caused during such time sometimes is increased to an non-negligible level. In the worst case, the amount of deviation reaches a level which cannot be canceled with the response rate of the track-on circuit 2 when the tracking servo is changed over from its off state to its on state. In such case, zero-crossing of the traverse signal of the tracking error signal TE through a ground line GL does not occur during the time t3, as shown in FIG. 2, thus occasionally producing track count errors by a track counter, not shown, during track jump time. In this case, track addresses are read after the time point tu to find that the target track has been passed over, so that the time points ts-tu are repeated once or several times by track jump in the reverse direction before reaching the target track. The result is the prolonged overall accessing time at the time of track jump.
In addition, with the magneto-optical disc apparatus, the laser power under the recording mode is approximately ten times that under the reproduction mode, such that the signal level of respective signals outputted by the light receiving unit is changed proportionately.
Of course, the dynamic range for which sufficient precision may be realized in an integrated circuit (IC) executing the processing of the WPP circuit 1 is the broader the higher the source voltage.
However, in a portable magneto-optical disc apparatus, it is difficult to raise the source voltage to 3 V or higher, such that a lower voltage is preferentially used. Consequently, emphasis is placed on coping with the recording/reproducing modes in the magneto-optical disc apparatus such that the processing recision is neglected as a result of which a laser light spot on the magneto-optical disc is deviated from a data track (detracking).